As a positive electrode of an alkaline secondary battery, an unsintered electrode obtained by filling a paste containing an active material into a nickel foam substrate is used. In this case, since nickel hydroxide, which is an active material in a discharged state, is low in electric conductivity, cobalt oxyhydroxide (CoOOH) having high electric conductivity is often utilized as an electroconductive additive in order to increase the utilization factor of the active material. For example, a powder of cobalt oxide (CoO), cobalt hydroxide (Co(OH)2), or cobalt oxyhydroxide is added to a paste containing an active material. Cobalt oxide and cobalt hydroxide are oxidized into cobalt oxyhydroxide at the time of initial charge, and cobalt oxyhydroxide acts as an electroconductive additive. Further, the surface of the nickel hydroxide particles may be coated with a cobalt compound.
Cobalt oxyhydroxide is stable in a normal battery operating voltage range and insoluble in an alkaline electrolyte solution. However, when a positive potential approaches a negative potential by overdischarge of a battery or a battery is in a state of reverse charge, cobalt oxyhydroxide is reduced, resulting in a decrease in an oxidation number of cobalt (Co) and a reduction in electric conductivity. If cobalt oxyhydroxide is further reduced into cobalt hydroxide, it is eluted into an electrolyte solution, and does not serve as an electroconductive additive.
From this situation, an attempt to inhibit the reduction of cobalt oxyhydroxide is made. For example, in Patent Document 1 is proposed a constitution in which antimony or the like is added to an oxidized compound of cobalt. However, although a lot of substances to be added to an oxidized compound of cobalt are described in Patent Document 1, only a few substances of these substances, such as magnesium and aluminum, are actually evaluated for the change in the battery capacity as experimental data, and these descriptions do not allow presumption as to the characteristics exhibited by other substances.
In Patent Document 2 is described a nickel positive electrode for an alkaline secondary battery, which has nickel hydroxide and a divalent or higher cobalt compound which coats the surface of nickel hydroxide, and contains a compound such as calcium in a coating layer thereof. It is described that the oxygen generation overvoltage increases by containing a compound such as calcium.
On the other hand, in order to increase the capacity of the active material itself, an attempt to utilize γ type nickel oxyhydroxide is made. In a common alkaline secondary battery, nickel hydroxide (the oxidation number of Ni is 2) turns into β type nickel oxyhydroxide (the oxidation number of Ni is 3) by charging. The oxidation number of Ni of γ type nickel oxyhydroxide is about 3.5 to 3.7, and therefore a battery capacity can be theoretically increased.
However, since γ type nickel oxyhydroxide has a crystal structure in which alkali metal ions or water molecules are incorporated into a space between layers thereof, and has a larger volume than β type nickel oxyhydroxide, a swelling phenomenon of a positive electrode occurs if γ type nickel oxyhydroxide is produced in charging, and the positive electrode absorbs an alkali electrolyte solution retained in a separator to cause internal resistance of a battery to increase, and hence there is a problem that the cycle life of the battery is shortened.
In order to solve this problem, in Patent Document 3 is proposed a positive active material for an alkaline secondary battery principally made of Ni(OH)2, including nickel hydroxide having an oxidation number of Ni larger than 2 and including a higher cobalt compound containing a first alkali cation on the surface of nickel hydroxide, in which nickel hydroxide having an oxidation number of Ni larger than 2 contains a second alkali cation. In the examples, a nickel hydroxide compound containing about 0.7% by mass of lithium ions is disclosed. It is described that by the effect of inclusion of alkali ions in nickel hydroxide, the alkali cation concentration in the electrolyte solution is not changed even when γ type nickel oxyhydroxide is produced during charging.
Further, in Patent Document 4 is described active material particles made of composite particles, in which a surface layer principally made of a higher cobalt compound of cobalt having an oxidation number larger than +2 is provided on the surface of core layer particles principally made of higher nickel hydroxide, wherein lithium is solid solution in the active material particles in an amount which corresponds to 0.01 to 0.5 wt % of lithium as simple substance. It is described in Patent Document 4 that by incorporating lithium into a crystal of nickel hydroxide, the active material particles are stable even when the oxidation number of Ni is +3.2 to +3.4.